1. Field of the Invention:
The present invention relates to antenna systems. More specifically, the present invention relates to antenna beamforming networks for phased array antenna systems.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art:
Many millimeter wave radar and communication systems require wide bandwidth antennas. In addition, in certain conventional wideband millimeter wave systems a scanning beam is generated by an antenna mounted on a gimballed dish. Unfortunately, the mechanical scan effected by gimballed dish systems is relatively slow. Further, gimballed dish systems typically cannot simultaneously support multiple scanning beams.
In submillimeter wave applications scanning phased array antenna systems offer improved beam switching rates relative to gimballed dish systems. Further, beamforming networks in phased array systems allow these systems to provide multiple scanning beams. Within the beamforming network, a master signal is typically divided and sequentially shifted in phase by an array of phase shifting elements (phase shifters).
In such systems, the direction of the emitted beam varies with respect to changes in operating frequency. Thus, beamforming networks utilizing phase shifters are generally not well suited for wideband applications as a single beam direction can only support a limited frequency spectrum.
Further, the large signal loss (over 10 dB in certain applications) of millimeter wave phase shifters discourages the use of millimeter wave phase shifters even in relatively narrowband millimeter wave beamforming networks. Accordingly, the conventional approach of realizing beamforming networks by using an array of phase shifting elements is currently not practical in many wideband millimeter wave phased array antenna systems.
An antenna beamforming network for phased array antenna systems operative at radio frequencies has been developed without the utilization of phase shifting elements. Specifically, in the August, 1963 issue of Microwaves, page 82, J. McFarland and J. Ajioka disclose a true time delay, multiple-beam, constrained lens for feeding a planar antenna array. True time delay antenna beamforming networks are ostensibly realized without employing phase shifting elements. As a result, in an antenna system having a true time delay beamforming network, the resultant beam direction is independent of frequency variation. It follows that true time delay beamforming networks are well suited for inclusion in wideband antenna systems.
Unfortunately, a direct millimeter wave implementation of the true time delay beamforming network of McFarland-Ajioka poses a number of difficulties. For example, the Mcfarland-Ajioka beamforming network includes a number of transmission cables. At millimeter wavelengths, transmission cables typically introduce appreciable loss and are bandwidth limiting. It follows that in a millimeter wave version of the McFarland-Ajioka beamforming network, the lengths of transmission cables must be minimized. However this minimization of cable length inhibits mechanical flexibility by constraining elements within the McFarland-Ajioka beamforming network to be in close physical proximity. Limitations on mechanical flexibility may prevent inclusion of the beamforming network in certain airborne and conformal array applications and may compound heat dissipation problems. It follows that in certain applications a practical realization of the antenna array feed beamforming network disclosed by McFarland and Ajioka is effectively precluded, at millimeter wavelengths, by transmission cable loss characteristics and concomitant cable length requirements.
Hence, a need exists in the art for a wideband, true time delay beamforming network for millimeter wave phased array antennas.